Method of sulphur removing from pig iron



United States Patent METHOD OF SULPHUR REMOVING FROM PIG IRON GerhardDerge, Aspinwall, and Kenneth M. Goldman,

Pittsburgh, Pa., assignors to Carnegie Institute of Technology,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationJuly 8, 1952, Serial No. 297,782

9 Claims. (Cl. 7558) This invention relates to the removal of sulphurfrom pig iron. The presence of sulphur in pig iron is generally deemedundesirable whether the ultimate intended use of the pig is for themanufacture of'castings or as feed material for one of the steel-makingprocesses. The presence of sulphur in iron for castings tends to causeshrinkage, cracking and bleeding in the ultimate cast article. In pigiron to be used for refining into steel sulphur is undesirable becauseit is not readily removed by the usual methods of refining employed inthe manufacture of steel. The removal of sulphur in the steps whichtransform pig iron to steel accordingly requires additional processingtime or additional processing steps as well as additional fluxingmaterials beyond those normally employed in the manufacture of steel.The sulphur problem in pig iron and steel has become a problem ofincreasing importance over the years as the deposits of higher qualityores, coals and other raw materials have been depleted and the amount ofsulphur contamination has increased as it has become necessary to go tolower grade raw materials.

Several methods of removing sulphur from iron have been proposed butwithout completely satisfactory results. In general the processesheretofore proposed have been objectionable because of the high costsand the poor control of the process. For example, it has been proposedto externally desulphurize pig iron by the addition of sodium carbonate.However, the control is poor and it is virtually impossible to predicthow much sulphur will be removed in successive runs. Moreover, the slagresulting from the treatment is very corrosive on ordinary refractorymaterials and must be skimmed off of the metal before it can go to themixer. Another method which has been proposed is that of tumbling themolten pig iron with dry lime in a tumbling'barrel. This method, likethe sodium carbonate method, is hard to control and the results areunpredictable. A third method of desulphurizing pig iron was proposed byHeuer in Patent No. 2,193,593, issued March 12, 1940. According to theHeuer process it was proposed to remove the pig iron from the furnace toa special vessel where it was covered with a special slag, then treatedwith reducing agents producing an oxygen activity in the slag lower thanthe equilibrium conditions during the reaction of carbon and oxygen toform carbon monoxide. All of this was done as a batch treatment under areduced pressure atmosphere in the special ladle from which air wasevacuated. The Heuer process has not been generally adopted probablybecause it is a batch process, it requires special apparatus and aspecial slag and it requires holding the metal under reduced pressurefor extended periods of time.

It has generally been believed that sulphur could not successfully beremoved from pig iron without following one of the processes abovedescribed or some other equally elaborate process. It has also beengenerally believed that it was necessary to hold the metal beingdesulphurized in contact with the treating material over extendedperiods of time while the system attained or approached equilibrium. Thevarious desulphurizing processes which have been proposed haveaccordingly been batch processes.

We have found that all of these complex process steps and treatments canbe completely eliminated and that sulphur can be successfully andconsistently removed from pig iron by our invention at low cost andunder conditions which may be readily achieved and maintained at thecast house.

Our invention will best be understood by referring to a specific exampleof the manufacture of pig iron. An ordinary blast furnace is charged andoperated in the usual manner for the manufacture of pig iron. The pigiron is collected in the hearth of the furnace and tapped at regularintervals as it accumulates. The tapped iron is carried through arefractory trough and is usually accompanied by a certain amount ofmolten blast furnace slag despite the usual practice of tapping the slagthrough a separate cinder notch in the blast furnace. It ischaracteristic of our process that the blast furnace slag thus withdrawnis utilized in the desulphurization of the metal promptly after themetal and slag issue from the tap hole of the blast furnace. As themixed slag and molten ironissue from the tap hole and flow through therefractory trough we add metallic aluminum to the mixture. The slagbeing lighter than the molten metal rises to the top of the flowingstream in the refractory trough from which it is separated by a skimmerand carried to a separate vessel for disposal. We have found that theconditions of ebullition which take place in the refractorytroughsbetween the tap hole of the blast furnace and the skimmer promote ahighly satisfactory desulphurizing action upon the addition of aluminumand that a substantial portion of the sulphur transfers into the slagand is carried with the slag, away from the metal at the skimmer. Ifthere is not enough slag admixed with the pig iron as it leaves the taphole there will be some loss in overall efficiency of sulphur removal,however, this may be compensated for by introducing a proportion ofmolten slag into the metal from the cinder notch dependingupon thedegree of control which is desired.

We have found that by following this process we can remove substantialamounts of sulphur from the pig iron and produce a material, which insulphur content, is equal to that produced from the .highest grade rawmaterials. The addition of metallic aluminum to molten pig iron in thepresence of the slag accompanying the pig iron from the furnace causes amarked migration of the sulphur from the metal'finto the slag andresults in a metal of unexpectedly low sulphur concentration.

In the preferred commercial practice of our invention we add metallicaluminum; preferably in the form of wire or shot, into the molten pigiron in the refractory trough between the tap hole and the skimmer, atwhich point it is covered by a layer of blast furnace slag into whichthe sulphur migrates .under the influence of the aluminum. Thus treated,'the molten metal is almost immediately separated fromrthe slag as boththe metal and slag flow from the skimmer into separate ladles. It is atonce apparent that the desulphurizing reaction must be a reaction ofgreat rapidity in order to be effective in this practice. This in itselfis contrary to all of the beliefs heretofore held by those in the art.As we have pointed out earlier, it has been generally believed, e. g.Heuer, that in order to successfully desulphurize pig iron it wasnecessaryto hold the pig iron in contact wfith a special desulphurizingslag for extended periods 0 time. P

While We prefer to add the aluminum to the pig iron before the skimmerimmediately after it leaves the furnace, it may be added at other placesso long as the molten pig iron is in contact with molten blast furnaceslag at or shortly after the time the addition of aluminum is made, andis thereafter separated from the slag. For example, the aluminum mightbe added to the molten metal in the hearth just prior to casting.

It will be accordingly understood that while we have described apreferred practice, our invention may be otherwise practiced within theterms of the following claims.

We claim:

1. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of adding metallic aluminum to a movingstream of molten pig iron in the presence of molten blast furnace slag,and thereafter separating the slag from the molten iron.

2. The method of continuously removing sulphur from molten pig iron atthe blast furnace comprising the steps of substantially continuouslyadding metallic aluminum to a stream of mixed pig iron and blast furnaceslag as it leaves the blast furnace, and continuously separating theslag from the metal after the addition of aluminum.

3. The method of continuously removing sulphur from molten pig iron atthe blast furnace comprising the steps of tapping molten metal andaccompanying slag from the blast furnace into a skimmer, substantiallycontinuously adding metallic aluminum to the molten metal and slagbefore the skimmer, and continuously separating the slag from the metalafter the addition of aluminum.

4. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of tapping molten metal from the furnace ina moving stream, passing the stream of molten metal beneath a pool ofmolten slag, substantially continuously adding metallic aluminum to theportion of the molten stream of metal covered by the molten slag, andsubstantially continuously withdrawing molten metal from beneath themolten slag.

5. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of tapping molten metal and molten slagfrom the blast furnace in a moving stream, substantially continuouslyseparating the slag from the moving stream of metal all the whilemaintaining a layer of slag over the metal at the point of separation,and substantially continuously adding metallic aluminum to the portionof the metal stream covered by said layer of slag.

6. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of simultaneously tapping molten metal andmolten slag from the furnace, bringing the metal and slag into contactwith one another to form a moving stream of molten metal and moltenslag, adding metallic aluminum to the Ell molten metal, and separatingthe metal and slag into two streams after the aluminum addition.

7. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of tapping molten metal from the furnace ina moving stream, passing the stream of molten metal through beneath apool of molten blast furnace slag, substantially continuously addingmetallic aluminum to the portion of the molten stream covered by themolten slag, and substantially continuously replacing the molten slag inthe pool with slag from the blast furnace.

8. The method of removing sulphur from molten pig iron at the blastfurnace comprising the steps of withdrawing molten pig iron with admixedmolten blast furnace slag as a flowing stream from the furnace, addingmetallic aluminum to the stream, forwarding the stream with addedaluminum until substantial flotation of slag has been brought about, andseparating the molten slag from the pig iron.

9. The method of removing sulphur from molten pig iron at the blastfurnace by adding aluminum to the molten pig iron in the presence ofmolten blast furnace slag, and thereafter separating the slag from themolten 11011.

References Cited in the file of this patent UNITED STATES PATENTS2,204,813 Muskat June 18, 1940 2,397,737 Heuer Apr. 2, 1946 FOREIGNPATENTS 1,437 Great Britain of 1893 631,235 Great Britain Oct. 31, 1949

3. THE METHOD OF CONTINUOUSLY REMOVING SULPHUR FROM MOLTEN PIG IRON ATTHE BLAST FURNACE COMPRISING THE STEPS OF TAPPING MOLTEN METAL ANDACCOMPANYING SLAG FROM THE BLAST FURNACE INTO A SKIMMER, SUBSTANTIALLYCONTINUOUSLY ADDING METALLIC ALUMINUM TO THE MOLTEN METAL AND SLAGBEFORE THE SKIMMER, AND CONTINUOUSLY SEPARATING THE SLAG FROM THE METALAFTER THE ADDITION OF ALUMINUM.